This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.
The production of hydrocarbons from a reservoir oftentimes carries with it the incidental production of non-hydrocarbon gases and other materials. Such materials include acid contaminants such as hydrogen sulfide (H2S) and carbon dioxide (CO2), hydrocarbons having molecular weights outside of a target range, and other materials. For example, when H2S or CO2 are produced as part of a hydrocarbon gas stream, such as methane or ethane, the raw gas stream is sometimes referred to as “sour gas.” The H2S and CO2 are often referred to together as “acid gases.”
Processes have been devised to remove contaminants and other materials from a raw hydrocarbon streams. Such processes include distillation or absorption, for example, by a physical solvent or a chemically reactive species. All of these processes use a separation tower to remove target impurities, such as a contaminate gas, from a target material, such as a produced hydrocarbon.
For example, a separation tower may be used as a distillation column to separate materials by boiling point differences. In distillation, vapor flows from the bottom of the tower to the top of the tower, while liquid flows from the top of the tower to the bottom. As a result, the lower boiling point materials are concentrated in the top of the tower, while higher boiling point materials are concentrated in the bottom of the tower. A vapor is typically produced by a reboiler heating mixed liquids at the bottom of the tower. A portion of vapors flowing from the top of the tower are condensed and returned to the tower as a reflux flow.
In an absorption column, a solvent is contacted with a gas in a counter current flow, with the liquid solvent dropping through the rising gas. The products are a gas that has a substantially decreased concentration of a target material, and a liquid stream that has a substantially increased concentration of the target material.
Both absorption and distillation rely on mass transfer, which is accomplished by intimate contact between the vapor and liquid phases. In these processes, a tower containing a number of packed beds is often used to provide enhanced contact between the vapor and liquid phase over the simple dropping of the liquid phase through the vapor phase as droplets. Liquid distributors are placed over each packed bed to evenly distribute the liquid onto the bed, while allowing vapors to flow upward into a higher packed bed.
Conventional distributors rely on gravity flow from open troughs filled with the liquid. The troughs have metering orifices on the side walls or the bottom, meaning that liquid head above these orifices determines the flow rate. The liquid from each orifice typically discharges into small tube, or flow guide, which directs the stream of liquid to a discrete distribution point. Maintaining uniform flow from all of the distribution points is dependent on the type and design of the distributor and the ability to maintain a level orientation. These distributors are typically designed with a relatively low liquid head, e.g., about five to about eight centimeters, above the final metering element.
However, maldistribution of the liquid and vapor phases can occur. For example, maldistributions can be caused by fouling of the packing or a liquid distributor, mechanical failure, or operation under tilted or moving conditions. For example, in floating service, these distributors may provide uneven distribution to the top of the packing due to tilting and motion during operation, which can cause sloshing and splashing of the liquid inside the distributor. The maldistribution can result in substantial reductions in efficiency.
There have been alternatives suggested for use in services that are more prone to maldistribution. One approach uses a multiple spray nozzle apparatus over the cross-section of the tower. However, liquid distribution quality from spray nozzles may be poor since the spray patterns must overlap to achieve full coverage and fine droplets are often generated, which can be entrained with the vapor phase. Furthermore, spray nozzles rely on high pressure drop, e.g. >100 kPa, requiring external pumps for boosting liquid pressure. Also, spray nozzles have limited turndown and are prone to fouling.
Another alternative is a tube, or pipe, distributor. These distributors are comprised of a central pipe fed by a pressurized liquid line or an elevated reservoir of liquid. The central pipe is connected to multiple lateral pipes. Each lateral pipe has a plurality of orifices located on the bottom of the pipe for metering fluid discharge as a distribution point. These distributors can have several disadvantages. They are susceptible to fouling. Further, the lack of flow guides creates some uncertainty regarding location of liquid distribution to the top of the packing. The high liquid head may produce a liquid stream that can jet into the packing, which may lead to excessive foaming, splashing, and increased entrainment. Finally, high liquid velocities in the lateral pipes may trap vapor upon filling, since the bottom orientation of the metering orifice does not provide a vapor outlet. This may result in periods of uneven distribution if the vapor pocket occupies too much of the pipe cross-section.
One example of these types of systems is described in U.S. Pat. No. 6,149,136, which discloses a distributor for a packed liquid-vapor contact column. The liquid distributor includes a header tank, a liquid distribution plate having vapor riser passages, and a multiplicity of discrete reservoir cells each having an aperture to allow the flow of liquid into the column. Conduits are positioned for feeding liquid from the header tank into each cell. The liquid distribution plate has a uniform cross-cross structure with alternating vapor riser passages and reservoir cells of identical shape and size. The conduits may have two or more sections each feeding a discrete group of reservoir cells from a location of the header tank at an elevation spaced from that of the other sections. The distributor compensates for column sway or tilt when mounted on, for example, a ship.
U.S. Pat. No. 5,752,538 describes a liquid distributor for packed columns. The liquid distributor includes a baffle which extends over the cross section of a liquid-vapor contacting column and is suspended above a distributor floor. The baffle, having an open space, converges and mixes liquid flow falling from a liquid-vapor contacting zone above, before dispersing it to the distributor below. The open space of the baffle may also be fitted with a mixing device for further enhancement of liquid mixing.
U.S. Pat. No. 6,397,630 describes a floating marine structure. The floating structure has an oscillation angle (i) of not more than about 10 degrees, and bears an air distilling column with corrugated criss-cross packing. The undulating configuration of the corrugated strips of at least one pack is selected such that d/i<6, where d is the axis deviation of the cone spreading the liquid, when each pair of adjacent strips of the pack is inclined at an angle i in its general plane.
However, these techniques may not fully compensate for the loss of efficiency from maldistribution of liquids in a tilted or moving separation tower on a seagoing platform, such as a floating production, storage, and offloading (FPSO) platform. Further, none of these systems compensates for maldistribution of vapors in a separation tower.